This invention relates to the fabrication of superalloy articles, and, more particularly, to the preparation of cast superalloy articles from high melting temperature superalloy compositions.
Some of the most demanding applications for materials are found in aircraft gas turbine engines. The materials used in such engines must withstand high temperatures and stresses in adverse environments over long periods of time, and through many cycles of startup and shutdown. The performance objectives of the structures must be met with materials that are as light in weight as possible, because each extra pound of weight reduces the amount of payload that the aircraft can carry. Finally, the structures must be fabricated at as low a cost as possible, to maintain a low overall cost for the engine.
In the gas turbine or jet engine, air is drawn into the front of the engine and compressed by a series of compressor blades. The compressed air is mixed with fuel, and the mixture is ignited. The expanding combustion products turn a turbine that drives the compressor, and then expand out the rear of the engine to cause the engine to move forward. The principles of thermodynamics dictate that the temperature of the combustion as should be as high as possible to attain the highest possible performance and efficiency, but the available materials and construction techniques limit the temperatures at which the engine can operate. Thus, the development of efficient, powerful aircraft engines is reflected in a continuing effort to identify improved materials and methods of construction for the engines, which permit ever-higher operating temperatures.
The modern axial flow jet engine is generally tubular in shape, with the air drawn in the front end of the tube and the combustion gases expanded out the back end of the tube. The compressor and turbine are mounted on a rotating shaft that runs along the center of the engine, and the annular combustion chamber is disposed around the shaft. The engine must have the structural strength and rigidity to hold the operating components together and to maintain the rotating shaft in precise alignment so that the compressor and turbine can function properly. The structure must also resist the adverse effects of the high temperatures encountered in the gas flow path in the rear portions of the engine.
The conventional approach for manufacturing the portion of the structure of the engine subjected to high temperature is to fabricate the individual pieces of the load-bearing framework from a castable nickel-based or cobalt-based superalloy. A liner, generally made from a wrought superalloy, is placed between the cast frame structure and the hot combustion gas to resist the heat. The liner does not bear a substantial part of the structural loadings, but is present to protect the load-bearing structure from temperatures that might cause it to melt or degrade rapidly.
This type of frame construction results from the inability of readily castable superalloys to withstand the high temperatures of the combustion gas. Some superalloys are readily fabricated by melting and casting, while others can only be fabricated by this approach with great difficulty, if at all, because of the defects formed in components made of such alloys. This second class difficult-to-cast superalloys generally has higher operating temperature capabilities than the readily castable alloys, and it would be desirable to manufacture the frame components from the high melting temperature, difficult-to-cast superalloys, hereinafter also referred to as non-castable alloy or non-castable superalloys. Because these non-castable alloys often are also difficult to weld, even welded structural pieces made from these materials have not been practical in the past. The conventional approach has therefore required the separation of the load-bearing and the heat-resisting functions, and the provision of a separate part to accomplish each function. The separate parts add to the weight and complexity of the engine.
There is a need for an approach for improving the fabrication of aircraft jet engines to reduce their weight while retaining their high-temperature performance. The present invention fulfills this need, and further provides related advantages.